Acoustical vehicle horns commonly include a sheet metal housing having side and bottom walls and a circular mounting flange for receiving diaphragm and projector structure. The diaphragm and projector are usually connected to the housing flange simultaneously by bolts, rivets, welds or other conventional mechanical means.
An electromagnetic coil mounted within the housing adjacent the bottom wall internally receives a ferromagnetic pole piece fixed to the housing, and a ferromagnetic plunger attached to the diaphragm also extends into the coil having electric switch means associated therein wherein axial movement of the diaphragm operates the switch means for cycling energization of the coil to produce rapid axial vibration of the diaphragm to produce the desired audio frequencies and sound output level at the projector flare.
The characteristics of the audio signal generated are a result of the rate of diaphragm frequency vibration, and the displacement of the vibrating diaphragm and the sound output level is largely determined by the dimension of the air gap that exists between the innermost surfaces of the pole piece and plunger which are in opposed facing relationship. The dimension of the air gap between the opposed surfaces of the pole piece and diaphragm plunger must be such that these components do not touch, but an optimum electromagnetic induction takes place which adds to the vibration of the diaphragm and frequency of diaphragm vibration, and when "tuning" vehicle horns of this type the air gap dimension must be closely regulated in order to produce a desired sound output level. U.S. Pat. Nos. 3,412,219; 3,516,088; 4,116,158 and 4,135,473 disclose various approaches to adjusting the aforementioned air gap.
Automobile horns are mass produced and cost of manufacture is closely controlled and high production fabrication and assembly techniques are employed in their manufacture. The metal housing in which the electromagnetic coil is mounted is formed of drawn sheet metal, and this drawing fabrication step does not result in an accurately dimensioned housing due to variations in the metal ductility, and "spring back". Accordingly, due to the difficulty of maintaining accurate manufacturing tolerances electromagnetic vehicle horns will vary in the level of sound output if adjustment means are not utilized to overcome such tolerance variations, as described in the aforementioned patents.
Adjustment of the air gap between the pole piece and diaphragm plunger by the use of extra components, or machined surfaces, threads, etc. adds significant cost to the manufacture, and in U.S. Pat. No. 4,135,473 a method of horn adjustment is disclosed wherein the axial dimension of the housing is varied by deforming the housing sidewalls by regulating the dimension of a radially extending housing sidewall ridge. The disclosure of this patent permits the horn air gap to be adjusted after the horn components are completely assembled. However, the techniques disclosed in this patent are relatively expensive to practice due to the time involved, and the complexity of the apparatus employed.
It is an object of the invention to provide a method for assembling an electromagnetic horn wherein the critical dimensions of the horn housing are determined prior to assembly of the housing and diaphragm, and the horn housing is deformed to compensate for dimensional variations whereby the proper housing dimensions are achieved prior to assembly resulting in a uniform pitch audio output.
A further object of the invention is to provide a method of adjusting an electromagnetic audio alarm device utilizing a sheet metal housing wherein critical dimensions of the housing are determined, and modified if necessary, prior to assembly of a vibratable diaphragm to the housing.
A further object of the invention is to provide an electromagnetic horn construction wherein the horn utilizes a sheet metal housing having a diaphragm attached thereto defining a closed chamber, and wherein vent means are employed for the chamber which minimizes the likelihood of moisture entering the chamber.
In the practice of the invention a sheet metal horn housing of a cup configuration includes an electromagnetic coil mounted adjacent a bottom wall of the housing having a housing mounted pole piece located therein. The housing includes sidewalls terminating in an annular flange which defines a diaphragm mounting surface for receiving both a diaphragm and a spiral projector. The diaphragm includes a ferromagnetic plunger mounted thereon which extends into the coil, and opposed surfaces defined upon the pole piece and plunger form an air gap whose dimension is critical for producing a predetermined sound output level upon diaphragm vibration.
Prior to assembly of the housing and diaphragm, a measurement is taken between the housing diaphragm mounting surface and the pole piece gap surface, and another measurement is taken between the diaphragm mounting surface and its plunger gap defining surface. These two dimensions are compared, and an axial force is imposed upon the housing which axially deforms the housing bottom wall to an extent as determined by the difference of the aforementioned dimensions such that, upon assembly, a predetermined dimension is produced between the gap defining surfaces of the pole piece and plunger which results in a predetermined sound output level. Thus, the aforedescribed method of housing dimensioning overcomes any dimensional variations that may exist in the manufacture of the housing, or the manufacture and assembly of the diaphragm and its plunger.
Further, the horn of the invention utilizes unique venting means defined in the stem of the plunger by which the plunger is attached to the diaphragm. This arrangement permits the motor chamber of the horn to be vented into the projector reducing the likelihood of moisture or foreign matter entering the chamber through the vent. Additionally, the structure for attaching the diaphragm to the plunger includes a shaped washer which permits maximum clearance between the plunger stem and projector inlet resulting in high sound generation efficiency and transmission.